The present invention relates to systems, methods, and subassemblies for drilling oil, gas, and analogous wells, and more particularly to downhole failure detection.
When drilling a well it is desirable to drill as long as possible without wearing the bit to the point of catastrophic bit failure. Optimum bit use occurs when a bit is worn sufficiently that the useful life of the bit has been expended, but the wear is not so extensive that there is a high likelihood of mechanical failure which might result in leaving a portion of the bit in the well. Poor drilling performance, increased BHA (Bottom Hole Assembly) wear, and more frequent fishing jobs all result from continued drilling with bits which are in the process of mechanical failure. A system capable of detecting the early stages of bit failure, with the additional capability of warning the operator at the surface, would be of great value solving the problem of drilling to the point of catastrophic bit failure.
The innovations in this application provide a reliable, inexpensive means of early detection and operator warning when there is a roller cone drill bit failure. This system is technically and economically suitable for use in low cost rotary land rig drilling operations as well as high-end offshore drilling. The solution is able to detect impending bit failure prior to catastrophic damage to the bit, but well after the majority of the bit life is expended. In addition to failure detection, the innovative system is able to alert the operator at the surface once an impending bit failure is detected.
The problem of downhole bit failure can be broken down into two parts. The first part of the problem is to develop a failure detection method and the second part of the problem is to develop a method to warn the operator at the surface. Several approaches for detecting bit failure have been considered.
It appears that some work has been done on placing sensors directly in the drill bit assembly to monitor the bit condition. There is some merit in placing sensors in the bit assembly, but this methodology also has some distinct disadvantages. The main disadvantage is the necessity of redesigning every bit which will use the method. In addition to being costly, each new bit design will have to accommodate the embedded sensors which might compromise the overall design. A second disadvantage arises from the fact that sensor connections and/or data transmission must be made across the threaded connection on the bit to a data processing or telemetry unit. This is difficult in practice.
In any system that uses electronic components there must be a power source. In many downhole tools disposable batteries are used to power electronics. Batteries have the desirable characteristics of high power density and ease of use. Batteries that are suitable for high-temperature, downhole use have the undesirable characteristics of high cost and difficulty of disposal. Batteries are often the only solution for powering downhole tools requiring relatively high power levels.
The present application discloses a system and method of predicting and detecting downhole drill bit failure. In a sample embodiment, sensors are placed on a sub assembly. The sensors detect changes in induced bending and axial stresses which are related to roller cone bearing failure.
Each cone on a bit supports an average percentage of the total load on the bit. As one of the cones begins to fail, the average share of the total load on the bit that the failing cone can support will change. This change causes a change in the bending strain induced by the eccentric loading of the cones. In the preferred embodiment, an average value of strain for each of the strain gauges is computed, then divided by a similar average strain value for each of the other strain gauges. The ratio of the average strain in each strain gauge is used to predict bit failure. Bit failure indications are signalled to the surface operator.
The disclosed innovations, in various embodiments, provide one or more of at least the following advantages:
self calibrating: requires no pre-drilling data gathering with sensors to calibrate;
no special bit required;
design-independent prediction of bit failure;
adaptable to varying drilling conditions; and
early detection of bit failure reduces fishing,
early detection of bit failure permits greatly improved failure analysis (since bits can be pulled in time for informative routine analysis, without significant loss of running time) and hence rapid improvements in bit design.